1. Field of the Invention
The present invention relates generally to oil passage structures for a transmission and, more particularly, to an oil passage structure for a transmission including a valve housing chamber in which oil is reserved and a valve unit is housed.
2. Description of the Related Art
An automatic transmission is provided with a plurality of clutches and brakes, which are hydraulically controlled. A continuously variable transmission, a type of automatic transmission, is further provided with a primary pulley and a secondary pulley, and the pulleys are also hydraulically controlled. The automatic transmission is provided with a valve unit having a plurality of solenoid valves in such manner as to control supplying of working oil to the aforementioned clutches, the pulleys, etc. (Refer to, for example, Japanese Unexamined Patent Application Publication Nos. 60-84451 and 2002-21991). In addition, as described in Japanese Unexamined Patent Application Publication No. 2002-21991, the valve unit is immersed in working oil within a transmission case in many instances. By immersing the valve unit in working oil, air entrapment in a hydraulic circuit can be prevented, and each individual solenoid coil of the solenoid valves can be further cooled with working oil.
Since the valve unit is immersed in working oil, it is required to drain the working oil from the transmission case when the valve unit is demounted from the transmission case upon or during maintenance work and the like on a vehicle. In addition, in a case where the working oil is drained from the transmission case, it is further required to refill the transmission case with new working oil after the valve unit is re-mounted in the transmission case. Thus, drainage and refilling of working oil accompanying demounting and re-mounting of the valve unit has become a factor for increasing in man-hours and cost for maintenance work.